Medical ultrasound transducers send repeated acoustic pulses into a body with a typical pulse length of less than a microsecond, using a typical repetition time of 160 microseconds. This is equivalent to approximately a 12 centimeter penetration in human tissue. After sending each pulse, the systems listens for incoming body echoes. The echoes are produced by acoustic impedance mismatches of different tissues which enable both partial transmission and partial reflection of the acoustic energy.
As a result of the body's acoustic attenuation properties, echoes coming from greater depths are more attenuated than echoes coming from shallower depths. The signal decay rate in the human body is approximately 0.38 dB per microsecond. Modern ultrasound systems compensate for this signal decay rate by employing variable automatic gain controls which operate, for example, in proportion to the depth of a returned signal.
Referring to FIG. 1, a schematic of a prior art ultrasound transducer 8 is shown which includes a pulse generator 10 and a matching layer 12 for coupling ultrasound signals into a patient's body. An acoustic absorber backing 14 and support 15 are positioned behind pulse generator 10. Transducer 8 includes an application face 16 which is placed against the patient's body and from which the principal ultrasound pulses emanate. Pulse generator 10 also propagates pulses through rear face 18 into absorber backing 14. Echoes coming from support 15 are not desired because such echoes appear on the ultrasound display as noise artifacts. As a result, the attenuation rate of absorber backing 14 has to be high to prevent such echoes from appearing on a display screen.
When a pulse generator 10 is energized, a sound signal T is emitted in a forward direction and is reflected by body Tissue, whereas a sound signal B is transmitted in the rearward direction through absorber backing 14, reflected by support 15 and redirected in a forward direction. FIG. 2 is a schematic of reflected signal level vs. time and indicates the size of signal T as reflected from the body tissue vs. the size of the signal in absorber backing B as reflected from support 15. The difference in magnitude in signals T and B is achieved by making the attenuation of absorber backing 14 greater than the attenuation of sound in the body. Note that the sound in absorber backing 14 keeps bouncing back and forth between support 15 and pulse generator 10 until it is entirely absorbed.
It has been found, that when support 15 is attached to absorber backing 14, artifacts sometimes appear on the ultrasound display screen during imaging. This is particularly the case when transducer 8 is thin and when heat sinks (which are relatively thick) are used as backing support. A thin transducer is generally desired in order to make the overall transducer smaller and more easily handleable.
Due to the lessened thickness of absorber backing 14, the round trip attenuation of sound within absorber backing 14 is lower in thin aspect ratio transducers as compared to the thicker variety. This causes more sound energy to be available at pulse generator 10 and thereby causes display artifacts. The attenuation level of absorber backing 14 dictates a minimum thickness transducer 8 which can be made without artifacts. It has also been determined that the shape of a rear-attached heat sink, its placement with respect to absorber backing 14 and the method of mounting the heat sink all effect the amount of displayed artifact. It has been thought that such display artifacts were due to mechanical resonances in the transducer structure and, while various changes in geometry and attachment methods between the heat sink and support body 15 have been tried, some display artifact from rear-reflected signals still remains.
Further analysis of the sound reflective characteristics of transducer 8 in FIG. 1, especially when it is configured as a "thin" transducer, indicate a second source of reflected sound (i.e. signal S) which results from reflections from the back of support 15. Signal S is later in time than signal B due to the increased travel distance through support 15.
FIG. 3 is a schematic of signal level at pulse generator 10 as a function of time, considering signals T, B and S. The signal level T from body Tissue is the same as described for FIG. 2. The decay rate of signal B from absorber backing 14 is initially slightly higher than that shown in FIG. 2 because some of the initial pulse energy is transmitted into support 15. While signal S is in the support 15, it does not decay with time. Thus, signal S, which comes from the back surface of support 15, decays at a lower rate than signal B (which is entirely in absorber backing 14). This action causes the overall level of signal at pulse generator 10 to decay much more slowly. The knee of curve K corresponds to the time it takes for the first echo S from within support 15 to reach the face of pulse generator 10. That time is proportional to the thickness of acoustic absorber backing 14. The slope of curve portion S, i.e. the decay rate of echoes from within support 15, is determined by the ratio of the thickness of support 15 divided by the thickness of absorber backing 14. Thus, the thicker is support 15 and the thinner is absorber backing 14, the more display artifact is present. The geometry is also important. If support 15 is wider than the backing (as shown in FIG. 1), the slope of S is also reduced.
The patented prior art includes many teachings regarding attenuation of rear-projected acoustic signals. In U.S. Pat. No. 5,267,221, entitled "Backing for Acoustic Transducer Array", an acoustically absorptive backing is described which includes electrical through-conductors for connecting ultrasound transducers to electrical contacts on a support. The absorptive backing is required to both absorb and attenuate acoustic signals coupled from the transducers and from the electrical through-conductors. One version of the invention (see FIG. 5) illustrates a dual layer absorptive backing wherein the layer adjacent to the transducers is designed to absorb and attenuate acoustic energy from the transducers and the layer adjacent the support is designed to absorb and attenuate acoustic energy from the electrical through-conductors.
There is a need for a thin aspect ratio ultrasound transducer which exhibits both excellent heat dissipation properties and provides effective attenuation of rear-transmitted acoustic energy.